Sizing feeder ocpd and wire for 125amp rated subpanel/panel board in detached garage

Status
Not open for further replies.
N

nickmcburney

Member
Location
Grass Valley CA
Good evening,
I am a Nevada County Building Inspector in CA. I had this come up twice today in consecutive inspections. A detached garage is fed from the main service panel with a 2 pole 40 amp breaker and 2 #8cu plus neutral and ground. The panel board in the garage has 2 20 amp breakers for outlets and 1 15 amp breaker for lights. The branch circuits are sized correctly. I am being told that the ocpd and feeder wires from the house to the garage must be sized to the rating of the panel board in the garage which is 125 amps. At 100' distance from house to garage this is approx a #1 cu. This guy only installed a 3/4" conduit. Do I size to the load or the panel board rating? And if its to the panel board rating must is be equal to or greater to the panel board rating or must it be less then?
 
F

Fitzdrew516

Senior Member
Location
Cincinnati, OH
nickmcburney said:
Good evening,
I am a Nevada County Building Inspector in CA. I had this come up twice today in consecutive inspections. A detached garage is fed from the main service panel with a 2 pole 40 amp breaker and 2 #8cu plus neutral and ground. The panel board in the garage has 2 20 amp breakers for outlets and 1 15 amp breaker for lights. The branch circuits are sized correctly. I am being told that the ocpd and feeder wires from the house to the garage must be sized to the rating of the panel board in the garage which is 125 amps. At 100' distance from house to garage this is approx a #1 cu. This guy only installed a 3/4" conduit. Do I size to the load or the panel board rating? And if its to the panel board rating must is be equal to or greater to the panel board rating or must it be less then?
Click to expand...

Size your wire and conduit to the upstream breaker that feeds it. The rating on the panel board is just that - What the panel is rated to handle. With the info you have given it sounds like a proper installation. Only reason I would think you would want to upsize is for voltage drop, but a full 40 amps on #8's @ 240V is a VD of 2.6% and I'm assuming theyre not pulling anywhere near 40A.

- Drew
 
qcroanoke

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
Fitzdrew516 said:
Size your wire and conduit to the upstream breaker that feeds it. The rating on the panel board is just that - What the panel is rated to handle. With the info you have given it sounds like a proper installation. Only reason I would think you would want to upsize is for voltage drop, but a full 40 amps on #8's @ 240V is a VD of 2.6% and I'm assuming theyre not pulling anywhere near 40A.

- Drew
Click to expand...

I agree.
 
N

nickmcburney

Member
Location
Grass Valley CA
sizing feed ocpd and wire for 125 amp rated subpanel follow up

sizing feed ocpd and wire for 125 amp rated subpanel follow up

Fitzdrew516 said:
Size your wire and conduit to the upstream breaker that feeds it. The rating on the panel board is just that - What the panel is rated to handle. With the info you have given it sounds like a proper installation. Only reason I would think you would want to upsize is for voltage drop, but a full 40 amps on #8's @ 240V is a VD of 2.6% and I'm assuming theyre not pulling anywhere near 40A.

- Drew
Click to expand...

There is a ufer ground in the detached garage so I will have the neutral floating and bond the equipment grounds to the ufer in the garage.

So no where in the code does it say feeder wire and ocpd must meet or exceed the rating of the garage subpanel? The thinking in the office is that someone is going to add breakers to the garage panel up to its rating and change out the upstream ocpd which means the feeder wire is undersized and will fail. I know, that's a lot of ifs. Last question and thanks for all you input: Is there a load calc for a garage with 8 flourescent lights, 12 outlets and a 30 amp for an RV hook up?

thanks again.
 
N

nickmcburney

Member
Location
Grass Valley CA
Growing up in Grass Valley

Growing up in Grass Valley

good to hear. I worked out of Eureka for Humboldt County for 6 years and have been here for 5 months. Great department. Great place to live. Grew up in Santa Cruz. I've done alright so far.
 
F

Fitzdrew516

Senior Member
Location
Cincinnati, OH
nickmcburney said:
The thinking in the office is that someone is going to add breakers to the garage panel up to its rating and change out the upstream ocpd which means the feeder wire is undersized and will fail. I know, that's a lot of ifs.
Click to expand...

I see what you're saying, but I think that's over thinking it. With the same logic you could say "someone can change out this 15A breaker to a 20A breaker without changing the wire size". That's true, but you have to draw the line somewhere. The point being - There's not a way to truly make something idiot proof. We just hope that if someone doesn't really know what they're doing they hire an electrician to do the work for them.

nickmcburney said:
Last question and thanks for all you input: Is there a load calc for a garage with 8 flourescent lights, 12 outlets and a 30 amp for an RV hook up?

thanks again.
Click to expand...

I'm not all that great at residential load calcs because I am on the industrial/commercial side of things and my load calcs are done with actual hard numbers so maybe someone else can step in here. But If I were to make a couple assumptions they would be as follows:
Fluorescent lights - Assumed 4' fixture (2) lamp T8's = 64 watts each (1.25 demand factor) = 640W
12 general purpose outlets - 180 watts each @ 120v = 2,160w
30A 240v outlet - Assumed 24A draw = 5,760w
Total = 8560w or 35.6A @ 240v
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
Fitzdrew516 said:
Fluorescent lights - Assumed 4' fixture (2) lamp T8's = 64 watts each (1.25 demand factor) = 640W
12 general purpose outlets - 180 watts each @ 120v = 2,160w
30A 240v outlet - Assumed 24A draw = 5,760w
Total = 8560w or 35.6A @ 240v
Click to expand...



Usually no demand factor for resi lights, not considered continuous for the most part.

No 180W per general receptacles for resi either.
 
F

Fitzdrew516

Senior Member
Location
Cincinnati, OH
jumper said:
Usually no demand factor for resi lights, not considered continuous for the most part.

No 180W per general receptacles for resi either.
Click to expand...

See - I have no knowledge when it comes to residential haha. So is there a standard load for general receptacles for residential applications or how is that handled?
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
Fitzdrew516 said:
See - I have no knowledge when it comes to residential haha. So is there a standard load for general receptacles for residential applications or how is that handled?
Click to expand...

The only required loads are 1500VA for each SABC(min. 2) and 1500VA for laundry.

All other general receptacles are included in the general lighting load of 3VA per sq ft.
 
Last edited:
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The garage panel will need a main breaker or some other means of disconnect as required by 225.31 and 225.32.
 
K

klineelectric

Member
Location
FL
Occupation
electrical contractor
What about adding a backfed 40a breaker with breaker hold down in garage. Or will they assume that someone will replace that breaker also? With that logic they could replace a 125a with a 150a. Its not an inspectors job to assume. imo
 
Status
Not open for further replies.
Top